Oil or gas located in a subterranean formation can be recovered by drilling a wellbore into the formation. Accurate knowledge of borehole caliper can be advantageous in the drilling process. For example, in the wellbore completion process, knowledge of borehole caliper assists in determining accurate cement volume and for placing casing hardware such as centralizers. Knowledge of borehole caliper during drilling also gives access to information about how well the drilling process is controlled. For example, appearances of break-outs on some portion of the borehole wall are indicative of inappropriate borehole stress management. Another example is observation of a cork-screw shaped borehole that indicates non-optimal drilling. Borehole caliper knowledge also allows to better correct measurements affected by it such as electromagnetic imaging, electromagnetic resistivity measurements (e.g. propagation-style measurements), and density measurements among others.
In oil-based muds, micro-imagers are operated at high frequencies (100 s of kHz to 10 s of MHz), making the estimation of caliper complicated by the influence of formation resistivity and formation dielectric effects. The use of multiple frequencies has been proposed to compensate for standoff effects with the end goal of getting robust formation imagery. But the dielectric dispersion at high frequencies of oil-based muds and formations complicates greatly the applicability of such a method. Another approach is to put up several devices at different distances to the borehole wall. After estimation of formation properties, an estimation of mud properties and standoff is devised. This assumes that the tool is positioned in the borehole at the same location during the two times for which the two sensors are facing the same position on the borehole wall. This is usually not the case because of drilling dynamics, and this makes the proposed method very sensitive to tool eccentering and drilling dynamics.